Tudorache Roman 23

8/10/2019 Tudorache Roman 23

1/15

Acta Polytechnica Hungarica Vol. 7, No. 2, 2010

39

The Numerical Modeling of Transient Regimesof Diesel Generator Sets

Tiberiu Tudorache, Cristian Roman

Electrical Engineering Faculty, University Politehnica of Bucharest313 Splaiul Independentei, Sect. 6, Bucharest, Romaniae-mail: [email protected]

Abstract: This paper deals with the numerical modeling of a diesel generator set used as amain energy source in isolated areas and as a back-up energy source in the case ofrenewable energy systems. The numerical models are developed using a Matlab/Simulinksoftware package and they prove to be a powerful tool for the computer aided design ofcomplex hybrid power systems. Several operation regimes of the equipment are studied.The numerical study is completed with experimental measurements on a Kipor type diesel-electric generator set.

Keywords: Diesel generator set; numerical models; experimental analysis

1 Introduction

The increasing demand for energy, the continuous reduction in existent resourcesof fossil fuels and the growing concern regarding environmental pollution havecompelled mankind to explore new production technologies for electrical energyusing clean renewable sources such as wind energy, solar energy, etc.

Among the electric power technologies using renewable sources, those based onthe conversion of solar and wind energy are clean, silent and reliable, with low

maintenance costs and small ecological impact. Sunlight and the kinetic energy ofthe wind are free, practically inexhaustible, and involve no polluting residues orgreenhouse gases emission. Along with these advantages, however, electric power production systems using as primary sources exclusively solar and wind energy pose technical problems due to uncontrollable wind speed fluctuations and to theday night and summer winter alternations. As a consequence, in autonomousregimes, the power supply continuity of a local grid should be backed-up by otherreliable and non-fluctuant sources of primary energy, such as diesel generator sets.Such systems, designed for the decentralized production of electric power usingcombined sources of primary energy, are called hybrid systems.


2/15

T. Tudorache et al. The Numerical Modeling of Transient Regimes of Diesel Generator Sets

40

By combining several renewable energy sources (wind/PV/diesel), a hybridsystem may ensure increased reliability in the electric power supply to consumers,at optimum quality parameters, in the condition of a minimum required electric power during an imposed time period.The increased interest in using diesel generator sets as the main energy source inisolated areas or as an emergency source in the case of renewable-based powersystems can be observed by the great number of papers and studies carried out inthis area. The research conducted in this domain refers to aspects such as: islandoperations of diesel generator sets [1], simulations of diesel/pv/wind hybrid powersystems [2]-[3], numerical modelings using special regulation techniques such asneural networks and fuzzy logic [4]-[6], etc. Various aspects about theseregulation techniques can be found also in [7]-[8].

The topic proposed in this paper refers to the development of numerical modelsfor the simulation of the operation regimes of diesel generators integrated as a back-up energy source in hybrid power systems as back-up energy source. Thenumerical analysis is completed by experimental investigations on a Kipor typediesel generator.

2 Diesel Generator Sets

Diesel generator sets convert fuel energy (diesel or bio-diesel) into mechanicalenergy by means of an internal combustion engine, and then into electric energy by means of an electric machine working as generator.

2.1 The Main Parts and Characteristics of Diesel GeneratorSets

The main parts of a diesel generator are: the internal combustion engine, usuallyair- or water-cooled; the electric generator usually of synchronous type; themechanical coupling; the support chassis; the battery for generator start-up; thefuel tank; the starter motor; the command panel, etc.The main characteristics of a diesel generator set are: rated power, rated voltage,rated frequency, number of phases, etc.

The diesel generator sets are usually designed to run at 3000 rpm or 1500 rpm at afrequency of 50 Hz. The primary movers are internal combustion enginesequipped with mechanical regulators to keep the imposed speed, integrated in theinjection pump and adjusted to obtain an output frequency of about 52 Hz withoutload and 50 Hz for rated load.


3/15


4/15


42

In the case of power systems based on renewable energies, given the fluctuantcharacter of unconventional energy sources (e.g. wind or solar energy), dieselgenerator sets takes on particular importance, their role being to ensure thecontinuity of electric power for the local grid during periods when the renewablesources of energy become unavailable or insufficient.

3 The Numerical Modeling of a Diesel Generator Set

The analysis of the complex aspects of a diesel generator set requires thedevelopment of reliable numerical models that allow for simulation in differentoperation regimes, specifically in conditions as close as possible to the reality ofthe assembly internal combustion engine the synchronous generator.The synchronous generator represents a key component of a diesel generator set. Itconverts the mechanical power produced by the primary mover into electrical power. The numerical models that can be used in the study of the synchronousgenerator can be classified into circuit models and field models, with the onesmost used in electric drive systems being the circuit models.

From the circuit models category of the synchronous machine, the most used is theorthogonal model (d-q) that allows for the study of different operation regimes ofthe machine, specifically the study of the assembly machine the regulation

system.The orthogonal model (d-q) of the synchronous machine is based on thedecomposition of the three-phase machine in an equivalent bi-phase one, Fig. 1. Inthis way the phasors system specific to the three-phase machine is decomposedalong the longitudinal axisd and transversal axisq respectively.

Figure 1Transformation of a three-phase machine in an equivalent bi-phase one


5/15


43

The voltage equations of the synchronous generator written in a coordinate system qd situated at the electric angle with respect to the fixed coordinate system

d q is as follows:

ss

sss jdt d

i Ru +

+= , (1)

dt d

i Ru r r r r

+= , (2)

where su and r u are the stator and rotor voltage phasors,s R and r R are thestator and rotor resistances, si and r i are the stator and rotor current phasors,

s and

r are the stator and rotor magnetic flux phasors and is the

pulsation [9].

In order to study the operation of a diesel generator set either inisolated regime orin emergency regime, dedicated numerical models were developed using theMatlab/Simulink software package.

3.1 Generator Set Operating in Isolated Regime

The numerical modeling scheme used for the simulation of a diesel generator setin isolated regime is presented in Fig. 2, where we can distinguish: thegeneratorset , the electricload , the normally openswitch I1, and themeasurement blocks.The generator set block is composed of the diesel internal combustion enginemodel and the synchronous generator model, and of the voltage and speedregulation systems, Fig. 3. The main data of the simulated generator set are asfollows: rated power 4.5 kVA, rated voltage 400 V, rated frequency 50 Hz.The development of numerical models for studying the operation of the dieselgenerator set in continuous regime permitted us to estimate the answer of thegenerator in case of load variations.In order to exemplify the effectiveness of the numerical models, we simulated thetransient regime of the equipment in the case of a sudden connection of a resistiveload. The initial state of the generator set is of stand-by type with no-load.These simulation results emphasize also the capacity of voltage and speedregulation systems to keep the imposed values of generator speed and voltage.The mechanical power (per unit) initially developed by the combustion engine isvery small because the synchronous generator coupled with the engine is runningwithout load, Fig. 4a). After suddenly connecting the resistive load, themechanical power developed by the engine increases rapidly and afterapproximately 1.5 seconds, a new operation stabilized regime is reached.


6/15


44

During the transient regime, due to the sudden coupling of the load, the machinespeed (per unit) decreases abruptly, but for a very short period of time (~0.25seconds), after which it stabilizes at the imposed value as a result of the action ofthe speed regulation system, Fig. 4b).The voltage (per unit) at the generator terminals also demonstrates a significantdecrease for a short time, after which it stabilizes in less than 0.5 sec. due to theaction of the voltage regulation system, Fig. 4c).

Figure 2Simulink model of a generator set operating on a local grid

a)


7/15


45

b)

Figure 3Simulink model of a generator set with voltage and speed regulation systems;a) global scheme of the generator set; b) Diesel engine and regulation systems

0 0.5 1 1.5 2 2.5 3 3.5 40

0.2

0.4

0.6

0.8

1

1.2

Time [s]

P m e c [ p u

]

a)


8/15


46

0 0.5 1 1.5 2 2.5 3 3.5 40

0.2

0.4

0.6

0.8

1

1.2

Time [s]

S p e e d [ p u

]

b)

0 0.5 1 1.5 2 2.5 3 3.5 40

0.2

0.4

0.6

0.8

1

1.2

Time [s]

V o l t a g e [ p u

]

c)

Figure 4Time variation of generator set quantities during the transient regime after applying a resistive load;

a) mechanical power; b) rotor speed; c) output voltage


9/15


47

3.2 Generator Set Operating in Emergency Regime

In the case of important equipment or objectives, electric power consumers areusually grouped into vital consumers and non-vital ones. An electric powersecurity solution for the vital consumers is to back-up their supply by means of adiesel generator set. The installation of the diesel generator set should be done sothat during a blackout resulting from a grid fault, it is possible to keep connectedonly the vital consumers. In the case of wind and/or solar renewable powersystems, a black out could occur in the event of insufficient solar or wind power.

Such a situation is presented in Fig. 5, where we can identify the following blocks:the Grid (three-phase, 400 V, 50 Hz, which can be represented by a renewable power system), the Generator set (4.5 kVA), the Three-phase short-circuit block,

vital consumers (2 kW resistive), and non-vital consumers (10 kW resistive).

Figure 5Generator set operating as emergency power source

The two groups of resistive type consumers have the rated powers 10 kW (non-vital consumers) and 2 kW (vital consumers), and they are supplied from the grid.When a three-phase short-circuit occurs at the grid level, the normally closed breakers I1 and I2 cut the energy supply for the non-vital consumers (10 kW), andthe normally open breaker I3 connects the vital consumers (2 kW) to the dieselgenerator set.

The three-phase short-circuit occurs in our case after 4 seconds from thesimulation start. At time instant 4.01 seconds, the I1 and I2 breakers open, and attime instant 7.01 seconds, the I3 breaker closes in order to supply the vital


10/15


48

consumers. Thus the vital consumers remain unsupplied for 3 seconds, the periodnecessary for the diesel generator set to start-up. If the vital consumers cannottolerate short periods of time without electricity, then the diesel generator shouldwork in stand-by regime, being ready at any time to be connected to the load.Moreover, if the vital consumers are sensitive to voltage and frequency variations,which can occur during the commutation from the grid to the diesel generator set,a UPS system should be provided as a buffer until the extinction of the transientregime.

The system answer in the case of emergency is presented in Fig. 6, where we canidentify the time evolutions of mechanical power, rotor speed and output voltage(per unit).

When the short-circuit occurs (after 4 seconds from the simulation start), the

mechanical power produced by the generator set increases from a small value (no-load regime) and stabilizes at the value imposed by the regulation systems, Fig.6a).

The output voltage (Fig. 6c) demonstrates an significant decrease, but for a short period of time, after which it comes back very quickly at the rated value. Such avoltage drop can be prevented by means of a properly sized UPS system.

5 6 7 8 9 10 11 120

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Time [s]

P m e c

[ p u

]

a)


11/15


49

5 6 7 8 9 10 11 120.98

0.985

0.99

0.995

1

1.005

Time [s]

S p e e

d [ p u

]

b)

5 6 7 8 9 10 11 12 13 14 150

0.5

1

1.5

Time [s]

V o

l t a g e

[ p u ]

c)

Figure 6Time variation of generator set quantities when operating in emergency regime;

a) mechanical power; b) rotor speed; c) output voltage


12/15


50

4 Experimental Results

The goal of the experimental analysis of the paper was to register the transient phenomena after coupling a load to the diesel generator set initially operating instand-by regime.

The diesel generator set subject to the experimental tests is a Kipor type, rated power 4.5 kVA, rated voltage 230 V, rated frequency 50 Hz, equipped withautomatic start-up.

As presented in Fig. 7, the experimental set-up consists of: a diesel generator set, alaptop with a data acquisition card, conditioning circuits and transducers, loadresistance, an ampere-meter, a voltmeter and an on/off breaker.

Figure 7Experimental set-up used for investigations

Using a data acquisition card, we stored the time evolution of the output currentand voltage produced by the generator set after applying a resistive load at thegenerator terminals.

At the beginning, the diesel generator set was operating in no-load regime, theload current being null, the generated voltage 230 V and the frequency 51.6 Hz.


13/15


51

When operating on a local grid, the diesel generator set used for experiments isdesigned in such a way that starting up the machine under load condition is notrecommended. Thus, after the start-up of the diesel generator set, we closed theload circuit by acting on the on/off breaker, the rms value of the absorbed currentreaching 15.2 A. The power delivered to the resistive load is 3.35 kW. We cannotice in Fig. 8 that the approximate duration of the transient regime is 0.13 s.

By studying the experimental results illustrated in Fig. 9, we can observe that thefrequency of the voltage at the generator terminals presents a decreasing trendfrom 51.6 Hz in no-load operation to 51.02 Hz under load operation of 66% fromthe rated load of the generator (a specific trend for this type of equipment).

a)

b)

Figure 8Time variation of load current after applying a resistive load; a) global view; b) detail


14/15


52

a)

b)

Figure 9Time variation of voltage at generator terminals after applying a resistive load;

a) global view; b) detail

We can observe in Fig. 9 a voltage drop at the generator terminals of about 70%, but it happens for a short period of time, after which the voltage regulator bringsthe voltage back to the imposed value.

Conclusions

This paper deals with aspects specific to the operation of diesel generator setsworking both in isolated regime and as a back-up electric energy source for vitalconsumers who are usually supplied from the grid or from hybrid power systems based on renewable sources.

The study of diesel generator sets was treated both via numerical modeling andexperiments in laboratory.

The numerical models used for the study of the dynamic regimes of dieselgenerator sets were developed using a Matlab/Simulink software package. Via


15/15


53

numerical simulations, we determined the time variations of the main quantitiesuseful in the study and operation of diesel generator sets (mechanical power,voltage, speed). We should observe also the usefulness, efficiency and flexibilityof developed numerical models in the study of the behavior of power systems fordifferent topologies, charge levels, load types, etc.

The experimental measurements aimed to determine the dynamic response of thediesel generator set after a sudden load connection. The diesel generator set thatwas experimentally tested was a Kipor 6700 one, with a rated power of 4.5 kVA.

Acknowledgement

This work was supported by the Romanian Ministry of Education and Research, inthe frame of the research grant PNII, 21-075/2007.

References[1] R. J. Best, D. J. Morrow, D. J. McGowan, P. A. Crossley: Synchronous

Islanded Operation of a Diesel Generator, IEEE Transactions on PowerSystems, Vol. 22, pp. 2170-2176, 2007

[2] K. Karasavvas: Modular Simulation of a Hybrid Power System with Diesel,Photovoltaic Inverter and Wind Turbine Generation, Journal of EngineeringScience and Technology Review, Vol. 1, pp. 38-40, 2008

[3] T. Tudorache, A. Morega: Optimum Design of Wind/PV/Diesel/BatteriesHybrid Systems, MPS 2008

[4] W. Shi, J. Yang, T. Tang: RBF NN-based Marine Diesel Engine GeneratorModeling, Proc. of American Control Conference, 2005

[5] H. Wei, R. Min, T. Yingqi: A Fuzzy Control System of Diesel GeneratorSpeed, APPEEC 2009 Conference

[6] W. Shi: Multi-Neural Networks Control of Marine Diesel Engine GeneratorSet, Proc. of 4th International Conference on Fuzzy Systems andKnowledge Discovery 2007

[7] M. L. Tomescu, S. Preitl, R.-E. Precup, J. K. Tar: Stability AnalysisMethod for Fuzzy Control Systems Dedicated Controlling Nonlinear

Processes, Acta Polytechnica Hungarica, Vol. 4, No. 3, pp. 127-141, 2007[8] J. ilkov, J. Timko, P. Girovsk: Nonlinear System Control Using Neural

Networks, Acta Polytechnica Hungarica, Vol. 3, No. 4, pp. 85-94, 2006[9] C. Ghi: Electrical Machines (in Romanian), Matrix-Rom Publishing

House, Bucharest, 2005

Documents

Tudorache Roman 23